Which one of the following three forms is correct for a multiple choice question?

Do you prefer the blue dress? or the red shirt?

Do you prefer the blue dress or the red shirt?

Do you prefer the blue dress? or the red shirt.

The first form seems the most syntactically adequate, as both "blue dress" and "red shirt" are equal options of equal value in the sentence.

The second form also seems adequate, as the question mark is marking the entire sentence as a question. However, it is ambiguous whether "blue dress" and "red shirt" are two separate choices, or "the blue dress or the red shirt" is a single choice.

Finally, the third form is the one that represents the spoken form most accurately. However, it looks somewhat strange for a question sentence not to end with a question mark.

should commas and or be used together like that?
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jgauffinFeb 12 '13 at 7:39

I tend to use both as a slight pause can occur after the comma, though it's ok without the comma as well.
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amanda wittFeb 12 '13 at 7:42

ok. I've always used commas when there are more than two choices: do you prefer the blue dress, white pants or a red shirt? and or for just two choices: do you prefer the blue dress or a red shirt?. That's why I asked.
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jgauffinFeb 12 '13 at 7:44

I'm specially concerned with the third form, as that one represents the spoken form (I elevate my pitch after the first option, but not after the second one)
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Panda PajamaFeb 12 '13 at 8:11

@PandaPajama are you writing dialogue? Otherwise, why do you want to represent the spoken form? We don't normally write to represent the spoken form, any more than we speak to represent the written form; we do both to communicate as best we can in each medium.
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Jon HannaFeb 12 '13 at 10:54

The purpose of commas is not to suggest places where those reading aloud might want to pause, but to help readers understand the structure of a sentence. If they do not help the reader in this way, then they serve no purpose.

In a sentence such as ‘Do you prefer the blue dress, the black coat or the red shirt?’ the comma after ‘dress’ is what R L Trask called a listing comma. His advice would be not to put another after ‘coat’, because the word ‘or’ already indicates a further, and final, alternative. The same logic makes any comma in your example equally superfluous. As Trask says ‘Put a listing comma before and or or only if this is necessary to make your meaning clear.’

The kind of punctuation shown in the first and third versions of your example is distracting, confusing and unlikely to be used by any reputable publisher.

Finally, the third form is the one that represents the spoken form most accurately.

I disagree, it's normally clear from spoken English that the second option is also something you are being asked about, which is what a question mark represents.

Further, written and spoken language do not coincide perfectly. There are things we can do with one that we cannot with the other. (We cannot gesture with our hands on the page, we can't change typographic choices in speech, as obvious examples).

Where they do coincide is in their purpose; in this case, to ask a question. Most of the time when we use such a sentence in writing, our task is not to reflect speech, it's to find out whether the red dress or the blue shirt is preferred.

The two times when we do want to reflect speech, is in dialogue and transcriptions. Even then we don't want to reflect it perfectly; most people talk in a poor first-draft at best, not to mention all the ums, ehs, broken sentences and so on (if a journalist wants to portray someone as an idiot, they'll record their speech faithfully).

Here, while the first form hasn't been common in writing for hundreds of years, it might serve in dialogue, to reflect the second option coming after an initial question form:

"Do you prefer the blue dress? or the red shirt?"

Even then, we'd likely break the reported speech at that point, to reduce the strangeness, and further emphasise the second part coming as a later thought:

"Do you prefer the blue dress?" she asked, "Or the red shirt?"

The third form would still not be used, because the second part is still a question. Unless it wasn't, but again we'd probably not want to depend on the quoted dialogue alone:

"Do you prefer the blue dress? or the red shirt." It was clearly not a question, the red shirt was the answer she wanted from me.